Noise reducing device, electronic camera, and image processing program for reducing image noise by using blackout image

ABSTRACT

A noise reducing device captures image data obtained by capturing a field with an image capturing part and a plurality of blackout image data obtained by capturing the field with the image capturing part under a light shielded state. This device reduces non-correlative random noise in the plural blackout image data. With random noise reduced, fixed pattern noise appears more accurately in resultant as blackout image data B. This device reduces the fixed pattern noise in the image data by using this blackout image data B.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2005-129831, filed on Apr. 27, 2005, andNo. 2005-146749, filed on May 19, 2005, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a noise reducing device, an electroniccamera and an image processing program for reducing image noise by usinga blackout image.

2. Description of the Related Art

Fixed pattern noise generally appears in image data obtained throughlong-exposure shooting with an electronic camera. There is a knowndevice for removing this type of noise disclosed in Japanese UnexaminedPatent Application Publication No. 2000-125204.

This conventional device first prepares image data that is normallycaptured and blackout image data that is captured while a shutter isclosed. Next, it subtracts the blackout image data from the image datain each pixel, to common-mode reject the fixed pattern noise.

However, the blackout image data captured through the long-exposureshooting includes random noise in addition to the fixed pattern noise.Therefore, the aforesaid the conventional device has a problem in thesubtraction processing that the random noise is reversed in phase andadded to the image data, resulting in increasing noise in the image dataunintentionally.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a technique toreduce the influence of random noise for noise reduction using ablackout image. Hereinafter, the present invention will be described.

-   <<1>> A noise reducing device of the present invention includes an    input part, a blackout image processing part, and a noise reducing    part. The input part captures image data obtained by capturing an    optical image with an image capturing part and a plurality of    blackout image data obtained by capturing the optical image with the    image capturing part under a light shielded state. The blackout    image processing part reduces non-correlative random noise in the    plural blackout image data to generate blackout image data B. The    noise reducing part reduces noise in the image data according to the    blackout image data B.

The aforesaid plural blackout image data are obtained from the sameimage capturing part, so that they have similar fixed pattern noises.Therefore, the fixed pattern noises in the plural blackout image datahave high correlativity. On the other hand, the random noises occur inthe plural blackout image data at random. Therefore, the random noisesin the plural blackout image data have non-correlativity.

Accordingly, comparing the plural blackout image data makes it possibleto accurately discriminate the non-correlative random noise from thecorrelative fixed pattern noise. According to the present invention, theblackout image data B with the random noise reduced through thiscorrelativity discrimination is generated. Using the blackout image dataB as a reference, the fixed pattern noise in the image data is reduced.As a result, it is possible to appropriately reduce the fixed patternnoise in the image data while inhibiting an adverse effect of increasein the random noise.

-   <<2>> Preferably, the blackout image processing part detects a    non-correlative portion of the plural blackout image data according    to level variation therein. The blackout image processing part    obtains the blackout image data B by applying variation reduction to    the non-correlative portion of a composite image of the plural    blackout image data.-   <<3>> Preferably, the blackout image processing part detects a    micro-level portion in which levels of all the plural blackout image    data are equal to or lower than a predetermined threshold value. The    blackout image processing part obtains the blackout image data B by    applying variation reduction to the micro-level portion of a    composite image of the plural blackout image data.-   <<4>> Preferably, a total charge storage time for the plural    blackout image data is set shorter than that for the image data. In    this case, the blackout image processing part multiplies the    blackout image data B by a compensation gain which compensates gain    decrease of the fixed pattern noise ascribable to the set shorter    time.-   <<5>> An electronic camera of the present invention includes an    image capturing part, a light shielding mechanism, a control part,    and a noise reducing device. The image capturing part performs    photoelectric conversion. The light shielding mechanism has a    function of shielding the image capturing part from light. The    control part drives the image capturing part to capture an image of    a field by and generate image data. Further, the control part    drives, a plurality of times, the image capturing part shielded from    light by the light shielding mechanism to generate a plurality of    blackout image data. The noise reducing device is the device    described in the above <<1>>. This noise reducing device captures    the image data and the plural blackout image data to reduce noise in    the image data.-   <<6>> Another electronic camera of the present invention includes an    image capturing part, a light shielding mechanism, a control part,    and a noise reducing part. The image capturing part generates image    data by photoelectric conversion. The light shielding mechanism    shields the image capturing part from light. The control part drives    the image capturing part to capture an image of a field and generate    image data. Further, the control part drives the image capturing    part shielded from light by the light shielding mechanism to    generate blackout image data. The noise reducing part reduces fixed    pattern noise in the image data based on the blackout image data.

In particular, for generation of the blackout image data, theabove-described control part sets an image capturing condition underwhich a level difference between the fixed pattern noise and randomnoise increases.

On the other hand, in the conventional noise processing, it has beencommon knowledge for those skilled in the art that image capturingconditions for the blackout image data and for the image data are to beuniform as much as possible so as to equalize the amount of the fixedpattern noise in both of the data.

Accordingly, it has been preferable that the charge storage time for theblackout image data and that of the image data be set equal to eachother. Further, it has been preferable that the image data and theblackout image data be generated continuously in order to prevent agreat change in temperature of the image capturing part, as well as thatimaging sensitivities of both of the data be set equal to each other.

However, the present inventors conducted an experiment of imagecapturing of blackout image data, and have found that a ratio of fixedpattern noise and random noise changes through intentional changes inthe image capturing conditions. The present invention positivelyutilizes the finding. That is, on the contrary to the common knowledgeof those skilled in the art, the image capturing condition is changedfor generation of the blackout image data, thereby increasing a leveldifference between the fixed pattern noise and the random noise in theblackout image data.

Such increase in the level difference relatively increases the fixedpattern noise and relatively decreases the random noise. Therefore,making the level of the fixed pattern noise in the blackout image dataequal to the level of the fixed pattern noise in the image data canreduce the random noise in the blackout image data. As a result, itbecomes possible to reduce an adverse effect of the random noise whennoise in the image data is reduced by using the blackout image data.

-   <<7>> Preferably, the control part increases the level difference    between the fixed pattern noise and the random noise by setting a    charge storage time for the blackout image data longer than that for    the image data.-   <<8>> Preferably, the control part increases the level difference    between the fixed pattern noise and the random noise by setting a    signal gain (hereinafter, imaging sensitivity) of the blackout image    data higher than that of the image data.-   <<9>> Preferably, the control part has a function of adjusting    temperature of the image capturing part. The control part sets the    temperature in generating the blackout image data higher than in    generating the image data. This increases the level difference    between the fixed pattern noise and the random noise.-   <<10>> Preferably, the noise reducing part corrects a level    difference in the fixed pattern noise between the blackout image    data and the image data due to a change in the image capturing    condition. After the level correction, the noise reducing part    reduces the fixed pattern noise in the image data by subtracting the    blackout image data from the image data.-   <<11>> Preferably, the noise reducing part discriminates the fixed    pattern noise from the random noise in the blackout image data    according to a threshold level which is in accordance with the image    capturing condition of the blackout image data. The noise reducing    part reduces the fixed pattern noise in the image data according to    the discriminated fixed pattern noise.-   <<12>> An image processing program of the present invention causes a    computer to function as the input part, the blackout image    processing part, and the noise reducing part described in the above    <<1>>.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature, principle, and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts aredesignated by identical reference numbers, in which:

FIG. 1 is a block diagram showing an electronic camera 11;

FIG. 2 is a flowchart to describe the operation of the first embodiment;

FIG. 3 is a block diagram showing an electronic camera 11 s;

FIG. 4 is a flowchart to describe the operation of a second embodiment;

FIG. 5 is a chart showing the relation of the charge storage time vs.fixed pattern noise and random noise;

FIG. 6 is a chart showing the relation of imaging sensitivity vs. fixedpattern noise and random noise; and

FIG. 7 is a chart showing the relation of temperature vs. fixed patternnoise and random noise.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail based on the drawings.

First Embodiment

FIG. 1 is a block diagram showing an electronic camera 11.

In FIG. 1, a photographing lens 12 is attached to the electronic camera11. A lens control part 12 a performs focus driving, aperture control,and so on of the photographing lens 12. In an image space of thephotographing lens 12, a light receiving surface of an image sensor 13is disposed via a shielding mechanism 12 b. The shielding mechanism 12 bmay serve also as a mechanical shutter or an aperture, or may be adedicated shielding mechanism. The shielding mechanism 12 b has afunction of closing an incident light path of the photographing lens 12based on a signal from an image capturing control part 14 to shield thelight receiving surface of the image sensor 13 from light.

The image sensor 13 is driven by the image capturing control part 14.Image data outputted from the image sensor 13 is processed in a signalprocessing part 15 and an A/D converter 16, and thereafter istemporarily stored in a memory 17.

The memory 17 is connected to a bus 18. The lens control part 12 a, theimage capturing control part 14, a microprocessor 19, an imageprocessing part 20, a recording part 22, and a monitor display part 23are also connected to the bus 18.

An operation part 19 a such as a release button is connected to theaforesaid microprocessor 19. A recording medium 22 a is loaded in theaforesaid recording part 22.

Description of Operation of First Embodiment

FIG. 2 is a flowchart to describe the operation of the first embodiment.

Hereinafter, the operation description will proceed following the stepnumber shown in FIG. 2.

Step S1: When detecting a user's release instruction in the operationpart 19 a or the like, the microprocessor 19 instructs the imagecapturing control part 14 to start an image capturing operation. Theimage capturing control part 14 makes the image sensor 13 start thephotoelectric conversion of a subject image while keeping the shieldingmechanism 12 b open, and after the elapse of a preset charge storagetime, reads image data from the image sensor 13. This image data passesthrough the signal processing part 15 and the A/D converter 16 to bedigitalized and thereafter, is temporarily stored in the memory 17.

-   Step S2: The microprocessor 19 determines a schedule for generating    a plurality of blackout image data according to the charge storage    time for the image data and instructs the image capturing control    part 14 to generate the blackout image data.

For example, in a case where N (N≧2) pieces of the blackout image dataare to be generated, a charge storage time for one blackout image datais preferably set to 1/N of the charge storage time for the image data.

Further, for example, in a case where N pieces of the blackout imagedata are to be generated, a time reduction ratio of the total chargestorage time for the blackout image data is defined as R (0<R<1). Inthis case, the charge storage time for one blackout image data ispreferably set to R/N of the charge storage time for the image data.

Furthermore, for example, in a case where N pieces of the blackout imagedata are to be generated, a time extension ratio of the total chargestorage time for the blackout image data is defined as E (E>1). In thiscase, the charge storage time for one blackout image data is preferablyset to E/N of the charge storage time for the image data.

Further, for example, it is also preferable that part or all of N piecesof the blackout image data are intentionally set to be different in thecharge storage time. Especially in this case, by conducting equationapproximation or analysis such as regression analysis regarding therelation on the time axis between the different charge storage times andgenerated fixed pattern noises, it is possible to determine linear ornonlinear relation between the both. As a result, it is possible to moreaccurately estimate the fixed pattern noise corresponding to the chargestorage time for the image data.

-   Step S3: In response to the instruction for generating the blackout    image data, the image capturing control part 14 closes the shielding    mechanism 12 b to keep the light receiving surface of the image    sensor 13 in a blackout state.-   Step S4: The image capturing control part 14 drives the image sensor    13 in the blackout state and executes charge storage/image reading    at least twice according to the generation schedule. The plural    blackout image data generated at this time are stored in the memory    17 after sequentially processed in the signal processing part 15 and    the A/D converter 16.-   Step S5: The microprocessor 19 refers to pre-stored correspondence    relation based on the charge storage time for the blackout image    data and so on to determine set values L, J, K. These set values L,    J, K are set in the image processing part 20 as parameters.

The set value L is a threshold value for use in discriminating amicro-level part, in the blackout image data, which obviously includesno fixed pattern noise. Such a set value L varies depending on a noisecharacteristic, the charge storage time, imaging sensitivity, and thelike of the image sensor 13. Therefore, it is preferable toexperimentally find the set value L in advance in correspondence to thecharge storage time, the imaging sensitivity, and the like of theblackout image data.

Note that one of the following values is preferably adopted as the setvalue L.

(a) a lower limit threshold value of the fixed pattern noise

(b) an upper limit threshold value of random noise

(c) the lower value of the above (a) and (b)

Further, when the plural blackout image data are different in the chargestorage time or the imaging sensitivity, the set value L is preferablydetermined individually for each of the blackout image data.

The set value J is a threshold value for use in discriminating whetheror not there is signal level correlativity among the blackout imagedata. Such a set value J varies depending on the noise characteristic,the charge storage time, the imaging sensitivity, and the like of theimage sensor 13. Therefore, the set value J is preferably experimentallydetermined in advance in correspondence to the charge storage time, theimaging sensitivity, and the like of the blackout image data. As the setvalue J, it is preferable to adopt a value (such as a value according toa standard deviation α) statistically obtained from variation widths ofthe fixed pattern noises in the individual blackout image data (whoseaverage signal levels are made uniform by being multiplied by anormalization coefficient if they differ in the charge storage time).

Further, the set value K is a value indicating a virtual signal level inthe blackout image data B (to be described later) when there is noinfluence of the random noise. It is preferable to experimentally findthe set value K in advance in correspondence to the total charge storagetime, the imaging sensitivity, and so on of the plural blackout imagedata. Alternatively, a DC level or the like detected from the blackoutimage data obtained at Step S4 may be determined as the set value K.

-   Step S6: The image processing part 20 initializes a scan position    [i, j] of the blackout image data to [0, 0] and reads a value of the    scan position [i, j] of each of the plural blackout image data.-   Step S7: The image processing part 20 judges whether or not the    values of the scan position [i, j] in the plural blackout image data    are all equal to or less than the set value L.

If the values of the scan position [i, j] in all the blackout image dataare equal to or less than the corresponding set value L, the imageprocessing part 20 judges that the scan position [i, j] is a micro-levelpart not including fixed pattern noise and shifts its operation to StepS9.

Otherwise, the image processing part 20 shifts its operation to Step S8.

-   Step S8: The image processing part 20 calculates a level difference    (absolute value) of the scan position [i, j] among the plural    blackout image data. If the number of the blackout image data is    three or more, a difference between the maximum value and the    minimum value of the scan position [i, j], a standard deviation of    the values of the scan position [i, j], an average value of level    differences between the individual blackout image data, or the like    is preferably found as the level difference.

Next, the image processing part 20 judges whether or not the found leveldifference is less than the set value J.

Here, if the level difference is less than the set value J, the scanposition [i, j] is a highly correlative part, and thus can be judged asthe fixed pattern noise. In this case, the image processing part 20shifts its operation to Step S10.

On the other hand, if the level difference is equal to or less than theset value J, the scan position [i, j] is a non-correlative portion, andthus can be judged as the random noise. In this case, the imageprocessing part 20 shifts its operation to Step S9.

When the plural blackout image data are successively processed one byone, the first blackout image data has no comparison target for findingthe difference, and therefore, the image processing part 20 preferablyshifts its operation to Step S10 unconditionally. In this case, thejudgment at Step S8 is started from the second blackout image data.

Further, as for the scan position whose value is determined as beingequal to or less than the set value L in one blackout image data ormore, the operation may shift to Step S9 from Step S7, skipping thejudgment at Step S8. Such an operation can reduce the processing time.

-   Step S9: The image processing part 20 substitutes the set value K in    the scan position [i, j] of the blackout image data B (preferably    one of the blackout image data in the memory 17 for saving memory    capacity). After this processing, the operation shifts to Step S11.-   Step S10: The image processing part 20 adds the values of the scan    position [i, j] in the plural blackout image data to find an    addition value and stores the addition value in the scan position    [i, j] of the blackout image data B.-   Step S11: The image processing part 20 judges whether or not the    scan of the blackout image data has been completed. If the scan has    not been completed, the operation shifts to Step S12. On the other    hand, if the scan has been completed, the operation shifts to Step    S13.-   Step S12: The image processing part 20 goes to the next scan    position [i, j]. The image processing part 20 reads values of the    new scan position [i, j] from the plural blackout image data. After    this processing, the image processing part 20 returns its operation    to Step S7.-   Step S13: The image processing part 20 determines a compensation    gain of the blackout image data B based on the sum total S of charge    storage times BT of the blackout image data and on a charge storage    time PT of the image data. If the level of the fixed pattern noise    exhibits a linear change with respect to the charge storage time,    the compensation gain may be determined as PT/S multiple. If the    change is nonlinear, the compensation gain corresponding to S and PT    (or BT and PT) may be found.

The image processing part 20 multiplies the blackout image data B by thefound compensation gain to make the level of the fixed pattern noisesubstantially uniform between the blackout image data B and the imagedata.

-   Step S14: In each pixel, the image processing part 20 subtracts the    blackout image data B from the image data in the memory 17. By this    subtraction processing, the image data with reduced fixed pattern    noise is obtained.

Effects etc. of First Embodiment

As has been described above, in the first embodiment, the pluralblackout image data are generated.

The fixed pattern noises in the plural blackout image data have highcorrelativity because of regular appearance thereof. On the other hand,the random noises in the plural blackout image data havenon-correlativity because of irregular appearance thereof. Therefore,applying variation reduction to the part having non-correlativity amongthe plural blackout image data as in the first embodiment makes itpossible to obtain the blackout image data with reduced random noise.Reducing image noise based on this blackout image data realizes thereduction in the fixed pattern noise with less influence of the randomnoise.

Further, in the first embodiment, the micro-level part where all theplural blackout image data have micro level is detected. The micro-levelpart is a part which cannot include the fixed pattern noise that isliable to increase level. Such a micro-level part is a part includingonly the random noise and can be considered as a part having a demeritrather than a merit in the removal of the fixed pattern noise.Therefore, applying variation reduction to the micro-level part makes itpossible to further reduce the influence of the random noise. As aresult, the reduction in the fixed pattern noise with less influence ofthe random noise is realized.

Since the influence of the random noise can be thus reduced, a tolerableamount of the random noise in the blackout image data is larger than ina conventional art. This can realize the setting of, for example,reducing the total charge storage time for the blackout image data. Inthis case, the electronic camera 11 can be freed from a photographingsequence of the blackout image data in a shorter time, which can realizethe electronic camera 11 superior in usability.

Supplemental Matter of First Embodiment

In the above-described first embodiment, the fixed pattern noise isremoved by the subtraction of the blackout image data from the imagedata. However, the first embodiment is not limited to this.

For example, the image processing part 20 evaluates the correlativityamong the plural blackout image data to detect a part where highlycorrelative fixed pattern noise appears (or removes a part wherenon-correlative random noise appears to thereby detect a part where thefixed pattern noise appears). Next, the image processing part 20 appliesthe noise reduction processing to a pixel position in the image datacorresponding to this appearance part, by referring to surroundingpixels or the like. Such processing makes it possible to accuratelydiscriminate the fixed pattern noise and the random noise in theblackout image data based on a difference in correlativity therebetween.As a result, this processing can also reduce the influence of the randomnoise given to the image data.

In the above-described first embodiment, the non-correlative randomnoise is reduced by substituting the set value K in the non-correlativeportion in the blackout image data. However, the first embodiment is notlimited to this.

For example, when three blackout image data or more are generated, it isalso possible to reduce the non-correlative random noise by imagecomposition in which median calculation (median) is performed on thethree blackout image data or more.

Further, for example, when three blackout image data or more aregenerated, the non-correlative random noise can be reduced also by imagecomposition in which averaging is performed by excluding pixel valuesgreatly different in level.

In the above-described first embodiment, the image data is firstgenerated and thereafter the blackout image data are generated. However,the first embodiment is not limited to this. Part or all of the pluralblackout image data may be generated before the image data is generated.Alternatively, part or all of the plural blackout image data may begenerated at a timing independent from the photographing of the imagedata, such as at power-on time. By such an operation, the photographingsequence of the blackout image data after the image data is photographedcan be reduced in time or eliminated.

In the above-described first embodiment, the electronic camera 11(including the noise reducing device) is described. However, the firstembodiment is not limited to this. For example, the noise reducingdevice may be structured independently from the electronic camera.Another possible example is to realize the above-described noisereduction processing (for example, shown in FIG. 2) by software andprepare an image processing program. Executing this image processingprogram by a computer can realize the noise reducing device on thecomputer.

The above first embodiment has described the case where the noisereduction processing is applied to the data generated by the electroniccamera. However, the first embodiment is not limited to this. Forexample, the above-described noise reduction processing can be executedby using image data and blackout image data generated by an imagegenerating device such as a scanner.

Noise reducing service in a server (image album server or the like) onthe Internet may be provided by using image data and blackout image datatransferred from a user.

Second Embodiment

FIG. 3 is a block diagram showing an electronic camera 11 s. Here inFIG. 3, the same reference numerals and symbols are used to designatethe same components as those of the first embodiment (FIG. 1), andrepeated description thereof will be omitted.

An image sensor 13 in the electronic camera 11 s has an integratedtemperature adjusting part 13 a. An image capturing control part 14drives the temperature adjusting part 13 a to control the temperature ofthe image sensor 13.

Note that the temperature adjusting part 13 a is preferably a mechanismcapable of heating/cooling such as a Peltier element or a Carnot heatengine. Further, as the temperature adjusting part 13 a, also preferableis a mechanism whose temperature can be raised, such as a resistor, anelectric circuit, or a light emitter (such as a light emitting elementfor a monitor screen of the electronic camera 11 s). The temperatureadjusting part 13 a may be formed as a circuit directly on asemiconductor substrate of the image sensor 13. Alternatively, thetemperature adjusting part 13 a may be integrated with a package of theimage sensor 13.

Especially in the case of the Peltier element, it is preferablyinterposed between the image sensor 13 and the monitor screen (lightemitting element), considering the arrangement relation of internalcomponents of the electronic camera 11 s. In such a structure, the useof the Peltier element to transfer heat of the light emitting element tothe image sensor 13 can increase the temperature of the image sensor 13in a short time.

Further, a temperature sensor 13 b is provided in the image sensor 13 orin the temperature adjusting part 13 a. Information on the temperaturedetected by the temperature sensor 13 b is transmitted to the imagecapturing control part 14.

Note that a monitor display part drives the monitor screen (includingthe light emitting element).

Description of Operation of Second Embodiment

FIG. 4 is a flowchart to describe the operation of the secondembodiment.

Hereinafter, the operation description will proceed following the stepnumber shown in FIG. 4.

-   Step S101: When the temperature adjusting part 13 a is an element    capable of selecting heating or cooling, such as the Peltier    element, the image capturing control part 14 preferably lowers the    temperature of the temperature adjusting part 13 a prior to an image    capturing operation of a still image. The image capturing control    part 14 detects the temperature at the time for photographing image    data by the temperature sensor 13 a to transmit this information to    a microprocessor 19.-   Step S102: The microprocessor 19, when detecting a user's release    instruction in an operation part 19 a or the like, instructs an    image capturing condition (charge storage time, imaging sensitivity,    and the like) of the image data to the image capturing control part    14. The image capturing control part 14 causes the image sensor 13    to start the photoelectric conversion of a subject image while    keeping a shielding mechanism 12 b open, and reads the image data    from the image sensor 13 after the elapse of the instructed charge    storage time.-   Step S103: In preparation for the generation of blackout image data,    the image capturing control part 14 drives the temperature adjusting    part 13 a to make the temperature adjusting part 13 a start raising    the temperature of the image sensor 13. At this time, preferably,    the image capturing control part 14 causes the temperature sensor 13    b to monitor the temperature and makes the temperature of the image    sensor 13 rise to a predetermined temperature. The temperature after    this rise is transmitted as information to the microprocessor 19    from the image capturing control part 14.-   Step S104: A signal processing part 15 amplifies the read image    according to the imaging sensitivity set for the image data. The    image data, after being amplified, is digitalized in an A/D    converter 16 and thereafter temporarily stored in a memory 17.-   Step S105: The image capturing control part 14 sets a charge storage    time for the blackout image data as long as possible, with the upper    limit being the custom-set time for the electronic camera 11 s (for    example, additional time allowed for noise removal) or the like. The    charge storage time is preferably set as long as possible within a    time range (experimental value or the like) causing no saturation of    the blackout image data (fixed pattern noise) inside the image    sensor 13. By such setting, the charge storage time for the blackout    image data is set longer than the charge storage time for most of    image data.-   Step S106: The image capturing control part 14 sets imaging    sensitivity (signal gain) of the signal processing part 15 as high    as possible within a range causing no signal saturation of the    signal processing part 15 and the A/D converter 16. By such setting,    the imaging sensitivity of the blackout image data is set higher    than imaging sensitivity of most of image data.-   Step S107: The image capturing control part 14 closes the shielding    mechanism 12 b to keep a light receiving surface of the image sensor    13 in a blackout state and a temperature risen state.-   Step S108: The image capturing control part 14 stores charges in the    image sensor 13 according to the charge storage time set at Step    S105 and thereafter reads the blackout image data from the image    sensor 13.-   Step S109: The signal processing part 15 amplifies the generated    blackout image data according to the imaging sensitivity set at Step    S106. The blackout image data after being amplified is digitalized    in the A/D converter 16 and thereafter is stored in the memory 17.

Preferably, as for the blackout image data at this stage, variationreduction is applied to a level range considered as random noise(non-fixed pattern noise) (generally, a low level range). Alternatively,a fixed value (a virtual signal level when the random noise is assumednot to occur) may be substituted in lieu of this level range. In thiscase, if a change of an image capturing condition does not cause aconspicuous change in the random noise, the level range considered asthe random noise may be constant irrespective of the change of the imagecapturing condition. On the other hand, if the change of the imagecapturing condition causes a change in the random noise, the setting ofthe level range considered as the random noise is preferably changedaccording to the change of the image capturing condition.

-   Step S110: The microprocessor 19 obtains information on the image    capturing condition of the blackout image data (here, charge storage    time, imaging sensitivity, temperature, and the like) from the image    capturing control part 14.

The microprocessor 19 refers to pre-stored correspondence relation basedon a change in the image capturing condition between the image data andthe blackout image data. By referring to this correspondence relation,the microprocessor 19 obtains information on a ratio of fixed patternnoise increase accompanying the change of the image capturing condition.

Note that this correspondence relation is prepared based on anexperiment or simulation in which the relation of the change of theimage capturing condition and the ratio of the fixed pattern noiseincrease accompanying the change is obtained.

-   Step S111: The microprocessor 19 selects a tone correction table for    use in level correction according to this incremental ratio to    transmit it to the image processing part 20.

The image processing part 20 level-corrects the blackout image dataaccording to the selected tone correction table. In this levelcorrection, the fixed pattern noise in the blackout image data ismultiplied by a correction gain that is inversely proportional to theincremental ratio. As a result, the level of the fixed pattern noise inthe blackout image data is lowered to become substantially equal to thelevel of the fixed pattern noise in the image data.

As for the blackout image data after the level correction, nonlinearvariation reduction is preferably applied to the level range (generallya low level range) considered as random noise (non-fixed pattern noise).Further, a fixed value (a virtual signal level when the random noise isassumed not to occur) may be substituted in lieu of this level range. Inthis case, the setting of the level range considered as the random noiseis preferably changed according to the change of the image capturingcondition and the level correction following the change.

-   Step S112: In each pixel, the image processing part 20 subtracts the    blackout image data, which has been level-corrected, from the image    data in the memory 17. By this subtraction processing, the fixed    pattern noise in the image data is reduced.

Effects etc. of Second Embodiment

Hereinafter, effects of the second embodiment will be described.

(Effect of Extending the Charge Storage Time)

FIG. 5 is a chart showing the actually measured relation of the chargestorage time vs. fixed pattern noise and random noise. As shown in FIG.5, the noise level of the fixed pattern noise prominently increases inaccordance with the increase in the charge storage time. On the otherhand, the noise level of the random noise does not noticeably changeeven when the charge storage time gets longer.

Therefore, a level difference between the fixed pattern noise and therandom noise can be increased by setting the charge storage time for theblackout image data longer than the charge storage time for the imagedata as in the second embodiment. The subsequent level correction of theblackout image data makes the noise level of the fixed pattern noisetherein substantially equal to the noise level of the fixed patternnoise in the image data. At this time, in accordance with an increasedamount of the level difference, the random noise in the blackout imagedata becomes smaller on the contrary.

As a result, less influence of the random noise is given in noiseremoval of the image data using the blackout image data which has beenlevel-corrected, so that it is possible to obtain a good noise removingeffect.

(Effect of Increasing Imaging Sensitivity)

FIG. 6 is a chart showing the actually measured relation of imagingsensitivity vs. fixed pattern noise and random noise. As shown in FIG.6, the noise level of the fixed pattern noise prominently increasesaccording to the increase in the imaging sensitivity. On the other hand,the noise level of the random noise does not noticeably increase evenwhen the imaging sensitivity gets higher.

Therefore, a level difference between the fixed pattern noise and therandom noise can be increased by setting the imaging sensitivity of theblackout image data higher than the imaging sensitivity of the imagedata as in the second embodiment. The subsequent level correction of theblackout image data makes the noise level of the fixed pattern noisetherein substantially equal to the noise level of the fixed pattern inthe image data. At this time, in accordance with an increased amount ofthe level difference, the random noise in the blackout image databecomes smaller on the contrary.

As a result, less influence of the random noise is given in the noiseremoval of the image data using the blackout image data which has beenlevel-corrected, so that it is possible to obtain a good noise removingeffect.

(Effect of Temperature Increase)

FIG. 7 is a chart showing the actually measured relation of temperaturevs. fixed pattern noise and random noise. As shown in FIG. 7, the noiselevel of the fixed pattern noise increases according to the increase intemperature. On the other hand, the noise level of the random noise doesnot noticeably increase even when the temperature gets higher.

Therefore, a level difference between the fixed pattern noise and therandom noise can be increased by setting the temperature at the time ofimage capturing of the blackout image data higher than the temperatureat the time of image capturing of the image data as in the secondembodiment. The subsequent level correction of the blackout image datamakes the noise level of the fixed pattern noise therein substantiallyequal to the noise level of the fixed pattern noise in the image data.At this time, in accordance with an increased amount of the leveldifference, the random noise in the blackout image data becomes smalleron the contrary.

As a result, less influence of the random noise is given in the noiseremoval of the image data using the blackout image data which has beenlevel-corrected, so that it is possible to obtain a good noise removingeffect.

(Synergistic Effect of a Plurality of Image Capturing Conditions)

Especially in the second embodiment, the changes in the plural imagecapturing conditions such as the charge storage time, imagingsensitivity, temperature, and the like work synergistically, so that itis possible to increase the level difference between the fixed patternnoise and the random noise in the blackout image data. Therefore, inaccordance with a synergistically increased amount of the leveldifference, the random noise in the blackout image data can be reduced.Therefore, it becomes possible to synergistically reduce the influenceof the random noise in noise removal of the image data using theblackout image data, which makes it possible to obtain a still betternoise removing effect.

Supplemental Items of Second Embodiment

In the above-described second embodiment, the fixed pattern noise isremoved by subtracting the blackout image data from the image data.However, the second embodiment is not limited thereto.

For example, in the above-described second embodiment, the leveldifference between the fixed pattern noise and the random noise in theblackout image data is increased in accordance with the change of theimage capturing condition. Therefore, the image processing part 20 setsa threshold level within this level difference according to the imagecapturing condition of the blackout image data, which enables even moreaccurate discrimination of a part where the fixed pattern noise appears.The image processing part 20 applies noise reduction processing to apixel position in the image data corresponding to this appearance part,by referring to surrounding pixels or the like. In this case, it is alsopossible to obtain a good noise removing effect with less influence ofthe random noise.

Further, in the above-described second embodiment, the plural imagecapturing conditions such as the charge storage time, imagingsensitivity, and temperature are changed when the blackout image dataare generated. However, the second embodiment is not limited to this. Atleast one of the charge storage time, imaging sensitivity, andtemperature may be changed.

Further, the image capturing conditions are not limited to these threekinds. Generally, any image capturing condition may be adopted providingthat a change thereof can increase the level difference between thefixed pattern noise and the random noise.

In the above-described second embodiment, the image data is firstgenerated and thereafter the blackout image data is generated. However,the second embodiment is not limited to this. The blackout image datamay be generated first. Alternatively, the blackout image data may begenerated at a timing independent from the generation of the image data(such as at power-on time) (in the above-described second embodiment, itis not necessary to make the image capturing conditions uniform, whichmakes it easier to generate the blackout image data at an independenttiming). Such an operation can eliminate the photographing sequence ofthe blackout image data after the image data is photographed.

The invention is not limited to the above embodiments and variousmodifications may be made without departing from the spirit and scope ofthe invention. Any improvement may be made in part or all of thecomponents.

1. A noise reducing device comprising: an input part that captures imagedata and a plurality of blackout image data, the image data beingobtained by capturing an optical image with an image capturing part, theplurality of blackout image data being obtained by capturing the opticalimage with said image capturing part under a light shielded state; ablackout image processing part that reduces non-correlative random noisein the plural blackout image data to generate blackout image data B; anda noise reducing part that reduces noise in said image data according tothe blackout image data B.
 2. The noise reducing device according toclaim 1, wherein said blackout image processing part detects anon-correlative portion of the plural blackout image data according tolevel variation therein and obtains the blackout image data B byapplying variation reduction to the non-correlative portion of acomposite image of said plural blackout image data.
 3. The noisereducing device according to claim 1, wherein said blackout imageprocessing part detects a micro-level portion of the plural blackoutimage data, and obtains the blackout image data B by applying variationreduction to the micro-level portion of the composite image of theplural blackout image data, the micro-level portion being a portion inwhich levels of all the plural blackout image data are equal to or lowerthan a predetermined threshold value.
 4. The noise reducing deviceaccording to claim 1, wherein when a total charge storage time for theplural blackout image data is set shorter than a charge storage time forthe image data, said blackout image processing part multiplies theblackout image data B by a compensation gain which compensates gaindecrease of a fixed pattern noise ascribable to the set shorter time. 5.An electronic camera comprising: the noise reducing device according toclaim 1; an image capturing part performing photoelectric conversion; alight shielding mechanism shielding said image capturing part fromlight; a control part that drives said image capturing part to capturean image of a field and generate image data, and drives said imagecapturing part shielded from light by said light shielding mechanism aplurality of times to generate a plurality of blackout image data,wherein said noise reducing device captures the image data and theplural blackout image data to reduce noise in the image data.
 6. Anelectronic camera comprising: an image capturing part generating imagedata by photoelectric conversion; a light shielding mechanism shieldingsaid image capturing part from light; a control part that drives saidimage capturing part to capture an image of a field and generate imagedata, and drives said image capturing part shielded from light by saidlight shielding mechanism to generate blackout image data; and a noisereducing part that reduces fixed pattern noise in the image dataaccording to the blackout image data, wherein for the blackout imagedata generation, said control part sets an image capturing conditionunder which a level difference between the fixed pattern noise andrandom noise increases.
 7. The electronic camera according to claim 6,wherein said control part increases the level difference between thefixed pattern noise and the random noise by setting a charge storagetime for said blackout image data longer than a charge storage time forsaid image data.
 8. The electronic camera according to claim 6, whereinsaid control part increases the level difference between the fixedpattern noise and the random noise by setting a signal gain (imagingsensitivity) of the blackout image data higher than that of the imagedata.
 9. The electronic camera according to claim 6, wherein saidcontrol part has a function of adjusting temperature of said imagecapturing part, and increases the level difference between the fixedpattern noise and the random noise by setting the temperature ingenerating the blackout image data higher than in generating the imagedata.
 10. The electronic camera according to claim 6, wherein said noisereducing part corrects a level difference of the fixed pattern noisebetween the blackout image data and the image data due to a change insaid image capturing condition, and reduces the fixed pattern noise inthe image data by subtracting the blackout image data from the imagedata after the level correction.
 11. The electronic camera according toclaim 6, wherein said noise reducing part discriminates the fixedpattern noise from the random noise in the blackout image data accordingto a threshold level which is in accordance with the image capturingcondition of the blackout image data, and reduces the fixed patternnoise in the image data according to the discriminated fixed patternnoise.
 12. An image processing program causing a computer to function asthe input part, the blackout image processing part, and the noisereducing part according to claim 1.